Methods and Systems for Advanced Ion Control for Etching Processes

ABSTRACT

A substrate is disposed on a substrate holder within a process module. The substrate includes a mask material overlying a target material with at least one portion of the target material exposed through an opening in the mask material. A plasma is generated in exposure to the substrate. For a first duration, a bias voltage is applied at the substrate holder at a first bias voltage setting corresponding to a high bias voltage level. For a second duration, after completion of the first duration, a bias voltage is applied at the substrate holder at a second bias voltage setting corresponding to a low bias voltage level. The second bias voltage setting is greater than 0 V. The first and second durations are repeated in an alternating and successive manner for an overall period of time necessary to remove a required amount of the target material exposed on the substrate.

CLAIM OF PRIORITY

This application is a divisional application under 35 U.S.C. 121 of U.S.patent application Ser. No. 14/932,458, filed on Nov. 4, 2015, thedisclosure of which is incorporated herein by reference in its entiretyfor all purposes.

BACKGROUND 1. Field of the Invention

The present invention relates to semiconductor device fabrication.

2. Description of the Related Art

Many modern semiconductor chip fabrication processes include generationof a plasma from which ions and/or radical constituents are derived foruse in either directly or indirectly affecting a change on a surface ofa substrate exposed to the plasma. For example, various plasma-basedprocesses can be used to etch material from a substrate surface, depositmaterial onto a substrate surface, or modify a material already presenton a substrate surface. The plasma is often generated by applyingradiofrequency (RF) power to a process gas in a controlled environment,such that the process gas becomes energized and transforms into thedesired plasma. The characteristics of the plasma are affected by manyprocess parameters including, but not limited to, material compositionof the process gas, flow rate of the process gas, geometric features ofthe plasma generation region and surrounding structures, temperatures ofthe process gas and surrounding materials, frequency and magnitude ofthe RF power applied, and bias voltage applied to attract chargedconstituents of the plasma toward the substrate, among others. It is ofinterest to understand and control some of the process parameters thatmay affect how the generated plasma interacts with the substrate,particularly with regard to generation and application of the biasvoltage. It is within this context that the present invention arises.

SUMMARY

In an example embodiment, a method is disclosed for plasma etching of atarget material in semiconductor device fabrication. The method includesan operation (a) for disposing a substrate on a substrate holder withina process module. The substrate includes a mask material overlying atarget material with at least one portion of the target material exposedthrough an opening in the mask material. The method includes anoperation (b) for generating a plasma in exposure to the substrate. Themethod includes an operation (c) for applying a bias voltage at thesubstrate holder at a first bias voltage setting corresponding to a highbias voltage level, for a first duration. The method includes anoperation (d) for applying a bias voltage at the substrate holder at asecond bias voltage setting corresponding to a low bias voltage level,for second duration, after completion of the first duration. The secondbias voltage setting is greater than 0 V. And, the second bias voltagesetting is sufficiently low to avoid ion-induced removal of the maskmaterial. The method includes and operation (e) for repeating operations(c) and (d) in an alternating and successive manner for an overallperiod of time necessary to remove a required amount of the targetmaterial exposed on the substrate.

In an example embodiment, a system is disclosed for plasma etching of atarget material in semiconductor device fabrication. The system includesa substrate holder configured to support a substrate in exposure to aplasma. The system includes an RF power supply connected to generate andtransmit RF signals to the substrate holder for generating a biasvoltage at the substrate holder. The RF power supply includes a first RFgenerator, a second RF generator, RF synchronization logic, andimpedance matching circuitry. The first RF generator and the second RFgenerator are configured to operate independently of each other. The RFsynchronization logic is configured to synchronize operation of thefirst RF generator and the second RF generator to enable generation ofthe bias voltage at the substrate holder as required for each of twoalternating process states.

In an example embodiment, a method is disclosed for plasma etching of atarget material in semiconductor device fabrication. The method includesan operation (a) for disposing a substrate on a substrate holder withina process module. The substrate includes a mask material overlying atarget material with at least one portion of the target material exposedthrough an opening in the mask material. The method includes anoperation (b) for generating a plasma in exposure to the substrate. Themethod includes an operation (c) for applying a bias voltage at thesubstrate holder at a first bias voltage setting corresponding to a highbias voltage level, for a first duration. The method includes anoperation (d) for applying a bias voltage at the substrate holder at asecond bias voltage setting corresponding to a low bias voltage level,for second duration, after completion of the first duration. The secondbias voltage setting is greater than 0 V. And, the second bias voltagesetting is sufficiently low to avoid ion-induced removal of the maskmaterial. The method also includes an operation (e) in which, for athird duration after completion of the second duration, zero biasvoltage is applied at the substrate holder. The method also includes andoperation (f) for repeating operations (c), (d), and (e) in a successivemanner for an overall period of time necessary to remove a requiredamount of the target material exposed on the substrate.

Other aspects and advantages of the invention will become more apparentfrom the following detailed description, taken in conjunction with theaccompanying drawings, illustrating by way of example the presentinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A shows a vertical cross-section through a portion of an examplesubstrate prepared for a plasma etching process, in accordance with someembodiments of the present invention.

FIG. 1B shows the vertical cross-section through the portion of theexample substrate from FIG. 1A, following performance of the plasmaetching process.

FIG. 2 shows an example substrate process module, in accordance withvarious embodiments of the present invention.

FIG. 3A shows an example plot of etch rate versus applied bias voltagefor a blanket silicon oxide mask material disposed on a substrate, inaccordance with some embodiments of the present invention.

FIG. 3B shows an example plot of etch rate versus applied bias voltagefor a blanket photoresist target material disposed on a substrate, inaccordance with some embodiments of the present invention.

FIG. 4A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention.

FIG. 4B shows an example plot of bias voltage versus time correspondingto the method of FIG. 4A, in accordance with some embodiments of thepresent invention.

FIG. 5A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention.

FIG. 5B shows an example plot of primary coil power versus timecorresponding to the method of FIG. 5A in combination with the exampleplot of bias voltage versus time from FIG. 4B, in accordance with someembodiments of the present invention.

FIG. 6A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention.

FIG. 6B shows an example plot of bias voltage generator RF signalfrequency versus time corresponding to the method of FIG. 6A incombination with the example plot of bias voltage versus time from FIG.4B, in accordance with some embodiments of the present invention.

FIG. 7A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention.

FIG. 7B shows an example plot of bias voltage versus time andcorresponding example plot of bias voltage generator RF signal frequencyversus time and corresponding example plot of primary coil power versustime corresponding to the method of FIG. 7A, in accordance with someembodiments of the present invention.

FIG. 8A shows a flowchart of an alternate method for providing advancedion control to improve etching of target material, in accordance withsome embodiments of the present invention.

FIG. 8B shows an example plot of bias voltage versus time andcorresponding example plot of bias voltage generator RF signal frequencyversus time and corresponding example plot of primary coil power versustime corresponding to the method of FIG. 8A, in accordance with someembodiments of the present invention.

FIG. 9 shows an example of the RF power supply in which a first RFgenerator and a second RF generator are used to supply RF signals to thesubstrate holder for generating the bias voltage at the substrate, inaccordance with some embodiments of the present invention.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the present invention. Itwill be apparent, however, to one skilled in the art that the presentinvention may be practiced without some or all of these specificdetails. In other instances, well known process operations have not beendescribed in detail in order not to unnecessarily obscure the presentinvention.

Methods and systems are disclosed herein for improvement in plasmaetching of material from a substrate in a semiconductor devicefabrication process. In an example embodiment, the term substrate asused herein refers to a semiconductor wafer. However, it should beunderstood that in other embodiments, the term substrate as used hereincan refer to substrates formed of sapphire, GaN, GaAs or SiC, or othersubstrate materials, and can include glass panels/substrates, metalfoils, metal sheets, polymer materials, or the like. Also, in variousembodiments, the substrate as referred to herein may vary in form,shape, and/or size. For example, in some embodiments, the substrate asreferred to herein may correspond to a 200 mm (millimeters)semiconductor wafer, a 300 mm semiconductor wafer, or a 450 mmsemiconductor wafer. Also, in some embodiments, the substrate asreferred to herein may correspond to a non-circular substrate, such as arectangular substrate for a flat panel display, or the like, among othershapes.

FIG. 1A shows a vertical cross-section through a portion of an examplesubstrate 101 prepared for a plasma etching process, in accordance withsome embodiments of the present invention. It should be understood thatthe substrate 101 is built up of multiple layers of different conductorand insulator/dielectric materials of specific shapes to form transistordevices and wires connecting various terminals and gates of thetransistors devices so as to form a prescribed integrated circuit. Forease of description, base 101A of the substrate 101 represents thiscollective build-up of multiple layers of different materials to aparticular point at which additional structures are to be formed.

FIG. 1A shows a layer of a target material 152 disposed over the base101A of the substrate 101, with a layer of a mask material 154 disposedover the target material 152. An opening 156 is formed through the maskmaterial 154 to expose an area of the underlying target material 152.With this configuration, a plasma etching process is performed to removea portion of the target material 152 exposed at the bottom of theopening 156.

FIG. 1B shows the vertical cross-section through the portion of theexample substrate 101 from FIG. 1A, following performance of the plasmaetching process. FIG. 1B shows removal of the portion of the targetmaterial 152 that was exposed through the opening 156 to the plasmaetching process. The opening corresponding to the removed portion of thetarget material 152 has an overall opening height 160 extending throughboth the mask material 154 and the target material 152 and an openingwidth 158. The opening width 158 may correspond to a critical dimension(CD) of an integrated circuit layout. The ratio of the opening height160 to the opening width 158 defines an aspect ratio of the opening.

In modern semiconductor device fabrication, high aspect ratio (HAR)etching has become a significant challenge. For example, in conductoretching processes, HAR etching of carbon is a particular challenge, butjust one of many extant HAR-related challenges. In HAR etchingprocesses, a trade-off is generally made between etch rate of the targetmaterial 152 and etching selectivity of the target material 152 relativeto the mask material 154 overlying the target material 152. Morespecifically, it may be necessary to sacrifice some amount of etchingselectivity of the target material 152 relative to the overlying maskmaterial 154 in order to increase etch rate of the target material 152.Also, in some etching processes, a higher bias voltage is applied at thesubstrate 101 level to attract charged constituents, e.g., ions, fromthe plasma toward the substrate 101 in a more direct manner in order toachieve a faster etch rate of the target material 152 andcorrespondingly better aspect ratio dependent etch (ARDE) of the targetmaterial 152. However, in some processes, etching selectivity of thetarget material 152 relative to the mask material 154 can drop offrapidly with application of increasing bias voltage at the substrate 101level.

In some etching applications, a high voltage bias pulsing (HVBP) etchingprocess is performed with low duty cycles, e.g., less than 50% highvoltage bias applied at the substrate 101 level versus zero bias voltageapplied, to improve etching selectivity of the target material 152relative to the mask material 154. However, experience indicates thatwith HAR geometries, the etch rate of the target material 152 becomesvery low (even approaching zero) as the aspect ratio increases when zerobias voltage is present at the substrate 101 level. Additionally,besides the trade-off between etch rate of the target material 152 andetching selectivity of the target material 152 relative to a maskmaterial 154, there can be other issues related to HVBP and/orcontinuous wave (CW) etching processes, such as difficulty with profilecontrol, hole distortion, and/or top clogging. For example, with thickercarbon mask material layers and smaller critical dimension for nextgeneration three-dimensional NAND devices, improved methods and systemssuch as those described herein are needed to meet process specificationsregarding etch rate, target versus mask selectivity, profile control,hole distortion, and/or top clogging, among others.

FIG. 2 shows an example substrate process module 100, in accordance withvarious embodiments of the present invention. The process module 100includes a substrate holder 102 configured to hold the substrate 101 inexposure to a plasma processing environment in which a plasma 104 isgenerated. The present disclosure primarily concerns apparatuses,systems, and methods by which one or more process parameters of biasvoltage, primary coil power, and bias voltage RF signal frequency aresystematically controlled to improve HAR etching of the target material152 without compromising the overlying mask material 154. To provideexample context, the process module 100 is depicted as an inductivelycoupled plasma (ICP) process module. However, it should be understoodthat in other embodiments the process module 100 can be defined as othertypes of process modules used in semiconductor fabrication.

The process module 100 is configured to provide for exposure of thesubstrate 101 to a plasma-based processing operation in order to modifycharacteristics of the substrate 101 in a prescribed and controlledmanner. The process module 100 includes a chamber 103 defined bysurrounding structures, including one or more wall structures 103A, abottom structure 1038, and a top structure 103C. In some embodiments,the top structure 103C is formed of a material through which RF signalscan be transmitted, such as quartz or ceramic, among others. The chamber103 can be formed of an electrically conductive material and have anelectrical connection to a reference ground potential 106.

The process module 100 includes a coil assembly 105 disposed above thetop structure 103C. An RF power supply 107 is connected to supply RFpower (RF signals) to the coil assembly 105 through a connection 109.The RF power supplied to the coil assembly 105 is referred to as primarycoil power herein. In various embodiments, the RF power supply 107includes one or more RF generators and associated impedance matchingcircuitry to provide for proper transmission of the RF power to coilassembly 105.

In various embodiments, the RF power supply 107 can include one or moreRF signal generators operating at one or more frequencies. Multiple RFsignal frequencies can be supplied to the coil assembly 105 at the sametime. In some embodiments, signal frequencies output by the RF powersupply 107 are set within a range extending from 1 kHz (kiloHertz) to100 MHz (megaHertz). In some embodiments, signal frequencies output bythe RF power supply 107 are set within a range extending from 400 kHz to60 MHz. In some embodiments, the RF power supply 107 is set to generateRF signals at frequencies of 2 MHz, 27 MHz, and 60 MHz. In someembodiments, the RF power supply 107 is set to generate one or more highfrequency RF signals within a frequency range extending from about 2 MHzto about 60 MHz, and generate one or more low frequency RF signalswithin a frequency range extending from about 100 kHz to about 2 MHz. Itshould be understood that the above-mentioned RF signal frequency rangesare provided by way of example. In practice, the RF power supply 107 canbe configured to generate essentially any RF signal having essentiallyany frequency as needed to generate the plasma 104 within the chamber103. Additionally, the RF power supply 107 can include frequency-basedfiltering, i.e., high-pass filtering and/or low-pass filtering, toensure that specified RF signal frequencies are transmitted to the coilassembly 105.

In some embodiments, the process module 100 includes a closable accessport 111, such as a gate valve or other component, through which thesubstrate 101 can be transferred into and out of the chamber 103. Theprocess module 100 also includes a number of process gas supply ports113A, 113B, 113C through which one or more process gas composition(s)can be supplied to the interior region of the chamber 103 overlying thesubstrate holder 102. During operation, a process gas supply 115operates to deliver the one or more process gas composition(s) throughone or more connection lines 117A, 1178, 117C to the process gas supplyports 113A, 1138, 113C, respectively, and RF power is delivered from theRF power supply 107 to the coil assembly 105, such that the RF powergenerates an electromagnetic field within a plasma generation regionbelow the top structure 103C and overlying the substrate holder 102 soas to transform the one or more process gas composition(s) within theplasma generation region into the plasma 104. Then, reactiveconstituents of the plasma 104, such as ions and/or radicals, interactwith portions of the exposed surfaces of the substrate 101.

The process module 100 includes a number of side vent structures 119through which gases and by-product materials can flow to an exhaust port121, which is connected to an exhaust module 123 configured to apply anegative pressure to the interior of the chamber 103 to facilitateexhaust of the used process gases and by-product materials. Also, insome embodiments, the substrate holder 102 is configured to receive biasRF power from a bias RF power supply 125 through a connection 127 toprovide for generation of a bias voltage on the substrate holder 102 inorder to attract ions from the plasma 104 toward the substrate holder102 and substrate 101 held thereon. In various embodiments, the RF powersupply 125 includes one or more RF generators and associated impedancematching circuitry to provide for proper transmission of the RF power tosubstrate holder 102.

In various embodiments, the RF power supply 125 can include one or moreRF signal generators operating at one or more frequencies. Multiple RFsignal frequencies can be supplied to the substrate holder 102 at thesame time. In some embodiments, signal frequencies output by the RFpower supply 125 are set within a range extending from 1 kHz (kiloHertz)to 100 MHz (megaHertz). In some embodiments, signal frequencies outputby the RF power supply 125 are set within a range extending from 400 kHzto 60 MHz. In some embodiments, the RF power supply 125 is set togenerate RF signals at frequencies of 2 MHz, 27 MHz, and 60 MHz. In someembodiments, the RF power supply 125 is set to generate one or more highfrequency RF signals within a frequency range extending from about 2 MHzto about 60 MHz, and generate one or more low frequency RF signalswithin a frequency range extending from about 100 kHz to about 2 MHz. Itshould be understood that the above-mentioned RF signal frequency rangesare provided by way of example. In practice, the RF power supply 125 canbe configured to generate essentially any RF signal having essentiallyany frequency as needed to generate a prescribed bias voltage at thesubstrate 101. Additionally, the RF power supply 125 can includefrequency-based filtering, i.e., high-pass filtering and/or low-passfiltering, to ensure that specified RF signal frequencies aretransmitted to the substrate holder 102.

Although the process module 100 depicts an example of an ICP processmodule, in various embodiments, the process module 100 can beessentially any type of process module used in semiconductor devicefabrication. For example, in some embodiments, the process module 100can be a capacitively coupled plasma (CCP) process module in which,instead of the coil assembly 105 used in the ICP process module, the CCPprocess module includes one or more electrodes disposed within thechamber 103, with RF power delivered to the one or more electrodes. Inthe CCP process module, the one or more electrodes can include one ormore of a top electrode (e.g., a showerhead electrode or solidelectrode, among others), a bottom electrode (e.g., an electrostaticchuck or substrate support, among others), and a side electrode (e.g., aperipheral ring-shaped electrode, among others), where the top, bottom,and side electrodes are configured around the plasma generation region.The RF power delivered to the one or more electrodes of the CCP processmodule is transmitted from the one or more electrodes through the one ormore process gas composition(s) present within the plasma generationregion to a reference ground potential, and in doing so transforms theone or more process gas composition(s) within the plasma generationregion into the plasma 104.

It should be understood that the ICP and CCP process module examplesmentioned above are discussed in a simplified manner for ease ofdescription. In reality, the process module 100, whether ICP, CCP, orsome other type, is a complex system that includes many components notdescribed herein. However, what should be appreciated for the presentdiscussion is that the process module 100, regardless of type, includesthe substrate holder 102 configured to hold the substrate 101 in asecured manner in exposure to the plasma 104 to enable processing of thesubstrate 101 to obtain a specific result. Examples of plasma processingoperations that may performed by the process module 100 include etchingoperations, deposition operations, and ashing operations, among others.

With regard to the substrate 101, the mask material 154 and targetmaterial 152 are selected such that the mask material 154 and targetmaterial 152 have different etching mechanisms. Specifically, the maskmaterial 154 is selected to be resistant to chemical etching, such thatetching of the mask material 154 is ion-driven. In some embodiments, themask material 154 is selected such that removal of the mask material 154in a given plasma etching process will primarily occur throughsputtering mechanisms. Examples of the mask material 154 include SiO₂,SiN, SiON, Si-ARC, among others. In contrast to the mask material 154,the target material 152 is selected to be subject to chemical etching,such that etching of the target material 152 is both chemically-drivenand ion-assisted. Etching of the target material 152 can be enhancedthrough ion interaction with the target material 152. Thus, ion-assistedchemical etching processes will be effective in removing the exposedportion of the target material 152. Examples of the target material 152include photoresist material, carbon material, doped carbon material,carbon doped material, silicon material, various metals (such astungsten, titanium, etc.), among others.

It should be understood that the mask material 154 is resistant tochemical etching and subject to ion-driven etching, while the targetmaterial 152 is subject to both chemical etching and ion-assistedetching. In some embodiments, the target material 152 will be composedof chemical components that are different than chemical componentspresent in the mask material 154. The mask material 154 and targetmaterial 152 can be essentially any materials that have theabove-mentioned etching characteristics in exposure to a particularplasma composition to be used.

The mask material 154 is also configured such that ions incident uponthe mask material 154 will need to have a threshold amount of kineticenergy in order to remove mask material 154 through a sputteringreaction. This threshold amount of ion kinetic energy required forsputtering of the mask material 154 corresponds to a threshold biasvoltage applied at the substrate 101, due to the direct correlationbetween the kinetic energy of the ions incident upon the mask material154 and the applied bias voltage at the substrate 101 level. During theplasma etching process, because the mask material 154 is not subject tochemical etching, when the applied bias voltage is below the thresholdbias voltage for removal of the mask material 154, there is essentiallyno removal of mask material 154 due to interaction of the plasmaconstituents (radicals/ions) with the mask material 154, regardless ofthe ion density present near the mask material 154. More specifically,when the applied bias voltage is less than the threshold bias voltagefor removal of the mask material 154, an increase in the ion densitywithin the plasma near the substrate 101 will not affect the etch rateof the mask material 154. However, when the applied bias voltage exceedsthe threshold bias voltage for removal of the mask material 154, theetch rate of the mask material 154 will increase in a substantiallylinear manner with increase in the applied bias voltage. And, when theapplied bias voltage exceeds the threshold bias voltage for removal ofthe mask material 154, an increase in the ion density within the plasmanear the substrate may cause an increase in the etch rate of the maskmaterial 154.

FIG. 3A shows an example plot of etch rate versus applied bias voltagefor a blanket silicon oxide mask material 154 disposed on a substrate,in accordance with some embodiments of the present invention. Theexample of FIG. 3A corresponds to exposure of the blanket silicon oxidemask material 154 to a plasma etching process in which the plasma isgenerated using a process gas mixture of 900 sccm (standard cubiccentimeters per minute) O₂ and 100 sccm COS. As shown in the example ofFIG. 3A, the threshold bias voltage for removal of the mask material 154is about 130 V. Below the threshold bias voltage of 130 V, the etch rateof the mask material 154 is essentially zero. Above the threshold biasvoltage of 130 V, the etch rate of the mask material 154 increases in asubstantially linear manner with the applied bias voltage.

Unlike the mask material 154, the target material 152 will etchchemically without bias voltage applied at the substrate 101 level.However, as the bias voltage is increased from zero, the target material152 etch rate will increase due to increased ion interaction with thetarget material 152. Also, an increase in the ion density within theplasma near the substrate will cause an increase in the etch rate of thetarget material 152. FIG. 3B shows an example plot of etch rate versusapplied bias voltage for a blanket photoresist target material 152disposed on a substrate, in accordance with some embodiments of thepresent invention. As with FIG. 3A, the example of FIG. 3B alsocorresponds to exposure of the blanket photoresist target material 152to the plasma etching process in which the plasma is generated using theprocess gas mixture of 900 sccm (standard cubic centimeters per minute)O₂ and 100 sccm COS. As shown in FIG. 3B, the target material 152 etchesthrough chemical reactions with constituents of the plasma with zerobias voltage applied at the substrate level. And, as the applied biasvoltage is increased from zero, the etch rate of the target material 152correspondingly increases. Therefore, as the applied bias voltage isincreased from zero to the threshold bias voltage for removal of themask material 154, the etch rate of the target material 152 willcorrespondingly increase while the etch rate of the mask material 154remains essentially zero.

Also, as the applied bias voltage increases, the etch front moves moredirectly toward the substrate due to the stronger attraction of the ionsfrom the plasma directly toward the substrate. Therefore, if the appliedbias voltage is set near the threshold bias voltage for removal of themask material 154, the integrity of the mask material 154 will bemaintained while the target material 152 will be etched with moredirectionality toward the substrate holder as compared to when theapplied bias voltage is set lower. With a higher applied bias voltage,the etch rate of the target material 152 will be higher and adversedistortion of the etched feature will be reduced. Given the foregoing,an etching process window includes an applied bias voltage rangeextending from 0 V to the threshold bias voltage for removal of the maskmaterial 154. Within this etching process window, the bias voltage canbe controlled to optimize ion-assisted etching of the target material152, particularly for HAR applications, without compromising integrityof the mask material 154. Methods are disclosed herein for exploitingthe different etch responses of the mask material 154 and targetmaterial 152 to attain advanced ion control in order to achieve a fasteretch rate of the target material 152, and achieve a higher etchingselectivity of the target material 152 relative to the mask material154, and achieve a better profile of the etched feature, and achieveless hole distortion of the etched feature.

FIG. 4A shows a flowchart of a method for providing advanced ion controlto improve etching of target material 152, in accordance with someembodiments of the present invention. With reference to FIG. 2, themethod includes an operation 401 in which a substrate 101 is disposed ona substrate holder 102 within a process module 100 to be subjected to aplasma etching process. As exemplified in FIG. 1A, the substrate 101includes the mask material 154 disposed over the target material 152,with portions of the target material 152 exposed through openings in themask material 154. The method also includes an operation 403 forgenerating a plasma in exposure to the substrate 101, with the plasmaincluding ions and reactive constituents such as radicals. The methodalso includes an operation 405 for applying bias voltage at thesubstrate holder 102 at a first bias voltage setting corresponding to ahigh bias voltage level. In some embodiments, the first bias voltagesetting is within a range extending from about 400 V to about 3000 V.The first bias voltage setting is greater than the threshold biasvoltage for removal of the mask material 154. The operation 405 isperformed for a first duration and corresponds to a first process state(A).

From the operation 405, the method proceeds with an operation 407 inwhich the bias voltage applied at the substrate holder 102 is set at asecond bias voltage setting corresponding to a low bias voltage level.The operation 407 is performed for a second duration and corresponds toa second process state (B). The second bias voltage setting is greaterthan zero. The second bias voltage setting is either 1) less than thethreshold bias voltage for removal of the mask material 154 such thatessentially no mask material 154 is removed during application of thesecond bias voltage setting, or 2) substantially near the threshold biasvoltage for removal of the mask material 154 such that an insignificantamount of the mask material 154 is removed during application of thesecond bias voltage setting. In some embodiments, the second biasvoltage setting is within a range extending from about 20 V to about 300V, depending on the threshold bias voltage for removal of the maskmaterial 154. In some embodiments, the high end of the range for thesecond bias voltage setting, i.e., for the low bias voltage level, isset at or just above the threshold bias voltage for removal of the maskmaterial 154. The method also includes an operation 409 for repeatingoperation 405 and 407 in an alternating and successive manner for anoverall period of time necessary to remove a required amount of theexposed target material 152.

FIG. 4B shows an example plot of bias voltage versus time correspondingto the method of FIG. 4A, in accordance with some embodiments of thepresent invention. The first bias voltage setting (H) is the high biasvoltage level of operation 405 and corresponds to the first processstate (A). The second bias voltage setting (L) is the low bias voltagelevel of operation 407 and corresponds to the second process state (B).The second bias voltage setting (L) is near the threshold bias voltage(TH) for removal of the mask material 154. According to operation 409,the bias voltage is successively alternated between the first processstate A and the second process state B. In some embodiments, theduration of the first process state (A) is less than the duration of thesecond process state (B). However, in some embodiments, the duration ofthe first process state (A) is greater than the duration of the secondprocess state (B). And, in some embodiments, the duration of the firstprocess state (A) is substantially equal to the duration of the secondprocess state (B). It should be understood that the mask material 154 ispreserved during the second process state (B), and the mask material 154is removed during the first process state (A). Therefore, in variousembodiments, the respective durations of process states (A) and (B) areset and controlled such that the mask material 154 will endure throughthe entirety of the plasma etching process to reach the end condition ofhaving removed the required amount of exposed target material 152, andsuch that removed portions of the mask material 154 will not interferewith or adversely impact continuation of the plasma etching processprior to reaching the end condition.

It should be understood that the first process state (A), in which thebias voltage setting is at the high bias voltage level, provides forrapid etch rate of the target material 152 and control of etched featuredistortion. It should also be understood that the second process state(B), in which the bias voltage setting is at the low bias voltage level,provides for control of etching selectivity of the target material 152relative to the mask material 154 while maintaining an effective etchrate of the target material 154. Additionally, it should be appreciatedthat the method of FIG. 4A provides for better overall etch rate of thetarget material 152 as compared to a different method in which the biasvoltage goes to zero in the second process state (B).

In some embodiments of the method of FIG. 4A, the respective durationsof process states (A) and (B) are set on the order of seconds. Theseembodiments are referred to as advanced mixed-mode pulsing (aMMP)methods for plasma etching. During the aMMP methods, the plasma ismaintained in a continuous manner as the bias voltage is changed in thedifferent process states (A) and (B). In various embodiments, the plasmais generated to enable realization of the differentiated etchingmechanisms of the mask material 154 and the target material 152 asdiscussed above. In some example embodiments, the plasma can begenerated using a process gas mixture that includes oxygen with an addedpassivation gas such as a carbon polymer and/or sulfur polymer. Forexample, in some embodiments, a process gas mixture of 1000 sccm O₂ and100 sccm COS is used to generate the plasma. It should be understood,however, that in other embodiments, the plasma can be generated usingessentially any other process gas mixture, so long as the differentiatedetching mechanisms of the mask material 154 and the target material 152are realized. Also, with reference to FIG. 2, in various embodiments,the flow rate of the process gas mixture within the chamber 103 is setas needed to enable generation and stabilization of the plasma. In someexample embodiments, a pressure within the chamber 103 is maintainedwithin a range extending from about 2 milliTorr (mTorr) to about 100mTorr. Alternately, in some example embodiments, the pressure within thechamber 103 is maintained within a range extending from about 10milliTorr (mTorr) to about 30 mTorr. Also, with reference to FIG. 2, insome example embodiments, the primary coil power supplied to the coilassembly 105 is within a range extending from about 100 Watts (W) toabout 5000 W.

In some embodiments of the method of FIG. 4A, the process states (A) and(B) correspond to respective portions of a same bias voltage pulsecycle, with the bias voltage pulsed at a frequency on the order of thefrequency of the RF signals transmitted to generate the bias voltage.These embodiments are referred to as modified high voltage bias pulsing(mHVBP) methods for plasma etching. Therefore, in the mHVBP methods,each bias voltage pulse cycle includes a high bias voltage durationcorresponding to process state (A) and a low bias voltage durationcorresponding to process state (B). In some embodiments, the mHVBPmethods include level-to-level pulsing of the bias voltage in afrequency range extending from about 10 Hertz (Hz) up to severalkiloHertz (kHz). Therefore, in the mHVBP methods, the respectivedurations of process states (A) and (B) are on time scales of less than0.1 millisecond (ms) and less than 100 ms. Thus, the respectivedurations of process states (A) and (B) in the mHVBP methods are about1000 times shorter than the respective durations of process states (A)and (B) in the aMMP methods. Also, it should be understood that duringthe mHVBP methods, the plasma is maintained in a continuous manner asthe bias voltage is changed in the different process states (A) and (B).

In the mHVBP methods, the first bias voltage setting corresponding tothe high bias voltage level in process state (A) is greater than thethreshold bias voltage for removal of the mask material 154. And, in themHVBP methods, the second bias voltage setting corresponding to the lowbias voltage level in process state (B) is greater than zero, but notsignificantly greater than the threshold bias voltage for removal of themask material 154, such that essentially no mask material 154 is removedduring the process state (B). During the mHVBP method, the bias voltageis maintained at a non-zero level. In other words, the mHVBP methoddisclosed herein does not include a state in which the bias voltage iszero, i.e., turned off. The mHVBP method represents a dual-level biasvoltage pulsing method for providing advanced ion control to enableetching of more target material 152 without losing more mask material154, while increasing etch rate of the target material 152 andincreasing etching selectivity of the target material 152 relative tothe mask material 154.

In some embodiments, the method of FIG. 4A can be implemented in amanner in which the process state (B) corresponds to a low bias voltagecontinuous wave (CW) process, with the process state (A) correspondingto addition of high bias voltage spikes on top of the low bias CWprocess. In this manner, process state (B) is actually a baseline of theprocess, and the process state (A) is generated by introducing high biasvoltage spikes is an additive manner. In some embodiments, the high biasvoltage spikes corresponding to process state (A) are introduced with aduty cycle of less than 10%, meaning that the duration of process state(A) is less than 10% of the sum of the durations of process steps (A)and (B). In these embodiments, the addition of the high bias voltagespikes may improve the etch profile control and provide other processbenefits related to high bias voltage, with minor loss of etchingselectivity of the target material 152 relative to the mask material154.

FIG. 5A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention. The method of FIG. 5A is anextension of the method of FIG. 4A. Specifically, the method of FIG. 5Aincludes operations 401, 403, and 409 as previously described withregard to FIG. 4A. However, in place of the operations 405 and 407 ofFIG. 4A, the method of FIG. 5A includes modified operations 405A and407A, respectively. In the operation 405A, for the first durationcorresponding to the first process state (A), the bias voltage isapplied at the substrate holder 102 at the first bias voltage settingcorresponding to the high bias voltage level, and a lower primary coilpower is supplied to the coil assembly 105. In some embodiments, thefirst bias voltage setting is within a range extending from about 400 Vto about 3000 V. The first bias voltage setting is greater than thethreshold bias voltage for removal of the mask material 154. In someembodiments, the lower primary coil power is within a range extendingfrom about 50 W to about 2000 W. The lower primary coil power in thefirst process state (A) corresponds to a lower ion density within theplasma. Therefore, in the first process state (A), when the maskmaterial 154 is subject to ion-induced sputtering, the ion densitywithin the plasma is lower so as to reduce the number of ions availableto cause the ion-induced sputtering of the mask material 154.

In the operation 407A, for the second duration corresponding to thesecond process state (B), the bias voltage is applied at the substrateholder 102 at the second bias voltage setting corresponding to the lowbias voltage level, and a higher primary coil power is supplied to thecoil assembly 105. The second bias voltage is set such that essentiallyzero ion-induced sputtering of the mask material 154 will occur. In someembodiments, the second bias voltage setting is within a range extendingfrom about 20 V to about 300 V, depending on the threshold bias voltagefor removal of the mask material 154. In some embodiments, the higherprimary coil power is within a range extending from about 2000 W toabout 5000 W. The higher primary coil power in the second process state(B) corresponds to a higher ion density within the plasma. Therefore, inthe second process state (B), when the mask material 154 is not subjectto ion-induced sputtering, the ion density within the plasma is higherso as to increase the number of ions available for etching of the targetmaterial 152. In some embodiments, the etch rate of the target material152 is directly proportional to the ion density within the plasma.

FIG. 5B shows an example plot of primary coil power versus timecorresponding to the method of FIG. 5A in combination with the exampleplot of bias voltage versus time from FIG. 4B, in accordance with someembodiments of the present invention. As shown in FIG. 5B, the primarycoil power changes from the lower primary coil power (LCP) in processstate (A) to the higher coil power (HCP) in process step (B) to thelower primary coil power (LCP) in process state (A), and so on. As shownin FIG. 5B, the method of FIG. 5A corresponds to pulsing of the primarycoil power and bias voltage in a synchronous inverted relationship toeach other.

In the method of FIG. 5A, the ion density within the plasma is increasedin accordance with having the bias voltage set below the threshold biasvoltage for ion-induced etching of the mask material 154. In thismanner, the etch rate of the target material 152 is increased by way ofthe increased ion density, while the mask material 154 does not etch dueto the bias voltage being near or lower than the threshold bias voltagefor ion-induced etching of the mask material 154. With the method ofFIG. 5A, the etch rate of the target material 152 is increased withoutfurther loss of mask material 154. Also, with the method of FIG. 5A, theetching selectivity of the target material 152 relative to the maskmaterial 154 is increased, and adverse distortion of the etched featureis decreased.

FIG. 6A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention. The method of FIG. 6A is anextension of the method of FIG. 4A. Specifically, the method of FIG. 6Aincludes operations 401, 403, and 409 as previously described withregard to FIG. 4A. However, in place of the operations 405 and 407 ofFIG. 4A, the method of FIG. 6A includes modified operations 405B and407B, respectively. In the operation 405B, for the first durationcorresponding to the first process state (A), the bias voltage isapplied at the substrate holder 102 at the first bias voltage settingcorresponding to the high bias voltage level, where the bias voltage isgenerated by transmitting RF signals of higher frequency to thesubstrate holder 102. In some embodiments, the first bias voltagesetting is within a range extending from about 400 V to about 3000 V.The first bias voltage setting is greater than the threshold biasvoltage for removal of the mask material 154. In some embodiments, theRF signals transmitted to generate the first bias voltage settingcorresponding to the high bias voltage level have a frequency of about13.56 megaHertz (MHz), which represents an example higher frequency. Itshould be understood, however, that in other embodiments, the higher RFsignal frequency used to generate the first bias voltage settingcorresponding to the high bias voltage level is within a range extendingfrom about 13 MHz to about 60 MHz. When the bias voltage is generatedusing RF signals of higher frequency, the resulting ion energydistribution (IED) near the substrate 101 will exhibit a single peakdistribution.

In the operation 407B, for the second duration corresponding to thesecond process state (B), the bias voltage is applied at the substrateholder 102 at the second bias voltage setting corresponding to the lowbias voltage level, where the bias voltage is generated by transmittingRF signals of lower frequency to the substrate holder 102. The secondbias voltage is set such that essentially zero ion-induced sputtering ofthe mask material 154 will occur. In some embodiments, the second biasvoltage setting is within a range extending from about 20 V to about 300V, depending on the threshold bias voltage for removal of the maskmaterial 154. In some embodiments, the RF signals transmitted togenerate the second bias voltage setting corresponding to the low biasvoltage level have a frequency of about 2 MHz, or 1 MHz, or 400kiloHertz (kHz), which represent examples of lower frequency. It shouldbe understood, however, that in other embodiments, the lower RF signalfrequency used to generate the second bias voltage setting correspondingto the low bias voltage level is within a range extending from about 400kHz to about 2 MHz. When the bias voltage is generated using RF signalsof lower frequency, the resulting IED near the substrate 101 willexhibit a double peak distribution, with one peak at the beginning ofthe distribution and one peak at the end of the distribution.

FIG. 6B shows an example plot of bias voltage generator RF signalfrequency versus time corresponding to the method of FIG. 6A incombination with the example plot of bias voltage versus time from FIG.4B, in accordance with some embodiments of the present invention. Asshown in FIG. 6B, the RF signal frequency used to generate the biasvoltage changes from the higher frequency (HF) in process state (A) tothe lower frequency (LF) in process step (B) to the higher frequency(HF) in process state (A), and so on. As shown in FIG. 6B, the method ofFIG. 6A corresponds to changing of the bias voltage generator RF signalfrequency and bias voltage in a synchronous relationship to each other.

The method of FIG. 6A takes advantage of change in the IED with changein frequency of the RF signals used to generate the bias voltage at thesubstrate 101. By changing the frequency of the RF signals used togenerate the bias voltage, it is possible to change the IED at thesubstrate 101. However, if the resulting bias voltage remains below thethreshold bias voltage for removal of the mask material 154, the changein IED will only effect etching of the target material 152. Use of lowerfrequency RF signals to generate the bias voltage corresponds to highervoltage peaks and higher average voltage. For example, due to changes inthe IED with RF signal frequency, if RF signals of a single frequencyare used to generate the bias voltage, it is possible to attain a higheretch rate and lower etching selectivity of the target material 152relative to the mask material 154 by using RF signals of lower frequencyto generate the second bias voltage setting corresponding to the lowbias voltage level. In this manner, because the low bias voltage is nearthe threshold bias voltage for etching of the mask material 154, themask material 154 will not etch regardless of the lower etchingselectivity of the target material 152 relative to the mask material 154caused by using RF signals of lower frequency to generate the biasvoltage. However, RF signals of higher frequency can be used to generatethe first bias voltage setting corresponding to the high bias voltagelevel to increase etching selectivity of the target material 152relative to the mask material 154, and thereby reduce loss of maskmaterial 154. It should be appreciated that by changing of the biasvoltage generator RF signal frequency and bias voltage in a synchronousrelationship to each other, a much wider process window is created whichwill extend to other process benefits.

FIG. 7A shows a flowchart of a method for providing advanced ion controlto improve etching of target material, in accordance with someembodiments of the present invention. The method of FIG. 7A is anextension of the method of FIG. 4A. Specifically, the method of FIG. 7Ais a combination of the methods of FIGS. 5A and 6A. The method of FIG.7A includes operations 401, 403, and 409 as previously described withregard to FIG. 4A. However, in place of the operations 405 and 407 ofFIG. 4A, the method of FIG. 7A includes modified operations 405C and407C, respectively. The operation 405C is a combination of theoperations 405A from FIG. 5A and 405B from FIG. 6A. In the operation405C, for the first duration corresponding to the first process state(A), the bias voltage is applied at the substrate holder 102 at thefirst bias voltage setting corresponding to the high bias voltage level,where the bias voltage is generated by transmitting RF signals of higherfrequency to the substrate holder 102, and a lower primary coil power issupplied to the coil assembly 105. The operation 407C is a combinationof the operations 407A from FIG. 5A and 407B from FIG. 6A. In theoperation 407C, for the second duration corresponding to the secondprocess state (B), the bias voltage is applied at the substrate holder102 at the second bias voltage setting corresponding to the low biasvoltage level, where the bias voltage is generated by transmitting RFsignals of lower frequency to the substrate holder 102, and a higherprimary coil power is supplied to the coil assembly 105.

FIG. 7B shows an example plot of bias voltage versus time andcorresponding example plot of bias voltage generator RF signal frequencyversus time and corresponding example plot of primary coil power versustime corresponding to the method of FIG. 7A, in accordance with someembodiments of the present invention. As shown in FIG. 7B, both theprimary coil power is changed between process states A and B and thebias voltage generator RF signal frequency is changed between states Aand B. In process state A, the bias voltage is high and the primary coilpower is low and the frequency of the RF signals used to generate thebias voltage is high. In process state B, the bias voltage is low andthe primary coil power is high and the frequency of the RF signals usedto generate the bias voltage is low.

FIG. 8A shows a flowchart of an alternate method for providing advancedion control to improve etching of target material, in accordance withsome embodiments of the present invention. The method of FIG. 8A is anextension of the method of FIG. 7A. The method of FIG. 8A includesoperations 401 and 403, as previously described with regard to FIG. 4A.Also, the method of FIG. 8A includes operations 405C and 407C, aspreviously described with regard to FIG. 7A. Additionally, the method ofFIG. 8A includes an operation 821, in which for a third duration, nobias voltage is applied at the substrate holder. In some embodiments,the higher primary coil power of operation 407C is supplied to the coilassembly 105 during operation 821. In some embodiments, the lowerprimary coil power of operation 405C is supplied to the coil assembly105 during operation 821. In some embodiments, the primary coil powersupplied to the coil assembly 105 during operation 821 is at a levelbetween the lower primary coil power of operation 405C and the higherprimary coil power of operation 407C. From the operation 821, the methodproceeds with an operation 823 for repeating operations 405C, 407C, and821 in a successive manner for an overall period of time necessary toremove a required amount of the exposed target material. In an exampleembodiment, the method of FIG. 8A is performed such that the operation405C includes applying a high bias voltage at about 1500 V for a firstduration of about 500 microseconds, and the operation 407C includesapplying a low bias voltage at about 100 V for a second duration ofabout 500 microseconds, and the operation 821 includes applying no biasvoltage for a duration of about 500 microseconds. It should beunderstood that in various embodiments, the respective durations ofoperations 405C, 407C, and 821 in the method of FIG. 8A can be the sameor different, depending on what is necessary to achieve a desiredprocessing result on the substrate.

FIG. 8B shows an example plot of bias voltage versus time andcorresponding example plot of bias voltage generator RF signal frequencyversus time and corresponding example plot of primary coil power versustime corresponding to the method of FIG. 8A, in accordance with someembodiments of the present invention. As shown in FIG. 8B, both theprimary coil power is changed between process states A and B and thebias voltage generator RF signal frequency is changed between processstates A and B. FIG. 8B also shows that no bias voltage is applied inprocess state C. In process state A, the bias voltage is high and theprimary coil power is low and the frequency of the RF signals used togenerate the bias voltage is high. In process state B, the bias voltageis low and the primary coil power is high and the frequency of the RFsignals used to generate the bias voltage is low. In process state C,the bias voltage is off, i.e., zero, and the primary coil power is kepthigh. However, in other embodiments, the process state C can have theprimary coil power set at any level necessary to achieve a desiredprocessing result on the substrate.

The method of FIG. 8A can be done as a modified HVBP process in whichthe multiple bias voltage levels, e.g., high, low, and zero, aregenerated in short pulses. By providing the zero bias voltage processstate (C) in operation 821, an additional “pure” radical flux isprovided at the top of the mask material and at the bottom of the etchedfeature, i.e., at the etch front. The “pure” radical flux on the top ofthe mask material can enhance polymer deposition to provide furtherprotection of the mask material. The amount of “pure” radical flux thatreaches the bottom of the etched feature is by diffusion, such that abigger sized etch feature will have more radical flux reach the bottometch front, while a smaller sized etch feature will have less radicalflux reach the bottom etch front. Considering deposition effects causedby radical flux reaching the etch front, in some embodiments, theprovision of “pure” radical flux due to zero bias voltage in operation821 can serve to offset ARDE results caused by ion-assisted etching, inwhich bigger sized etch features have a faster etch rate relative tosmaller sized etch features. The no bias process state (C) of operation821 can be used to improve intra-cell loading or iso-dense loading ofetch rate, etch profile, and CD difference, caused by small differencesin feature size of different cells, or caused by big differences infeature size between isolated and dense features.

In some embodiments, the methods of FIGS. 4A, 5A, 6A, 7A, and 8A can beimplemented using multiple RF generators within the RF power supply 125.FIG. 9 shows an example of the RF power supply 125 in which a first RFgenerator 801 and a second RF generator 803 are used to supply RFsignals to the substrate holder 102 for generating the bias voltage atthe substrate 101, in accordance with some embodiments of the presentinvention. Matching circuitry 807 is configured to control impedancematching so that the RF signals generated by the RF power supply 125 canbe transmitted effectively to the plasma loads within the chambers 103.Generally speaking, the matching circuitry 807 is a network ofcapacitors and inductors that can be adjusted to tune impedanceencountered by the RF signals in their transmission to the plasma loadswithin the chamber 103. In some embodiments, the first RF generator 801is configured as a pulsing generator, and the second RF generator 803 isconfigured as a pulsing generator. In some embodiments, the first RFgenerator 801 is configured to output RF signals of a first frequency,and the second RF generator 803 is configured to output RF signals of asecond frequency, where the first and second frequencies are different.

The RF power supply 125 includes RF synchronization logic 805 configuredto synchronize operation of the first RF generator 801 and the second RFgenerator 803, to enable generation of the bias voltage as required foreach of the alternating process states (A) and (B), as discussed above.In some embodiments, the RF synchronization logic 805 is configured toenable communication between the first RF generator 801 and the secondRF generator 803, vice-versa. In some embodiments, by way of the RFsynchronization logic 805, one of the first and second RF generators801/803 is configured to operate as a master RF generator, with theother of the first and second RF generators 801/803 configured tooperate as a slave RF generator. In these embodiments, the master RFgenerator operates to direct the slave RF generator when to output RFsignals to the substrate holder 102. In various embodiments, the RFsynchronization logic 805 is configured as a combination of hardware andsoftware. In some embodiments, the RF synchronization logic 805 can beimplemented in each of the first and second RF generators 801/803. In aparticular embodiment, the RF synchronization logic 805 is defined todirect one of the first and second RF generators 801/803 to operate in acontinuous wave mode, in which it generates RF signals and suppliesthose RF signal to the substrate holder 102 in a continuous manner.Also, in this particular embodiment, the RF synchronization logic 805 isdefined to direct the other of the first and second RF generators801/803 to operate in a pulsed manner, in which RF signals are generatedduring a high phase of a pulse for transmission in an additive manner inconjunction with the RF signals being supplied the other RF generator801/803 operating in the continuous wave mode.

As semiconductor device technology continues to develop, it is necessaryto etch more layers of alternating materials, such as alternatingSiO₂/SiN, or SiO₂/Si, among others, in order to form three-dimensionalstructures, particularly in NAND memory devices. Because of this,thicker and/or harder mask material is required, such as harder carbonmask for ON or OP etch. And, HAR etching of carbon (hole or slitfeatures) is a challenge due requirements for selectivity (greater than50-to-1 for target material-to-mask material, e.g.,carbon-to-SiO₂/SiN/SiON/Si-ARC), etch rate (depending on material andapplication, possibly greater than 300 nanometers/minute), and profilecontrol and hole distortion control (defined as minor axis/major axis ofan ellipse, with a target control of greater than 0.95 of ideal). Also,in performing the above-mentioned HAR plasma etching processes, an O₂plasma with some passivation gas, such as COS, SO₂, N₂, CH₄, amongothers, is an effective choice, with ion-assisted etch being the primaryetch mechanism. It should be understood, that the methods for providingadvanced ion control to improve etching of target material as discussedherein with regard to the methods of FIGS. 4A, 5A, 6A, 7A, and 8A can beused to perform difficult HAR etching processes, such as that mentionedabove. The methods disclosed herein provide for faster target materialetch rate, higher etching selectivity of target material relative tomask material, better profile control, and less hole distortion.

Although the foregoing invention has been described in some detail forpurposes of clarity of understanding, it will be apparent that certainchanges and modifications can be practiced within the scope of theappended claims. Accordingly, the present embodiments are to beconsidered as illustrative and not restrictive, and the invention is notto be limited to the details given herein, but may be modified withinthe scope and equivalents of the described embodiments.

What is claimed is:
 1. A system for plasma etching of a target materialin semiconductor device fabrication, comprising: a substrate holderconfigured to support a substrate in exposure to a plasma; aradiofrequency (RF) power supply connected to generate and transmit RFsignals to the substrate holder for generating a bias voltage at thesubstrate holder, the RF power supply including a first RF generator, asecond RF generator, RF synchronization logic, and impedance matchingcircuitry, wherein the first RF generator and the second RF generatorare configured to operate independently of each other, wherein the RFsynchronization logic is configured to synchronize operation of thefirst RF generator and the second RF generator to enable generation ofthe bias voltage at the substrate holder as required for each of twoalternating process states.
 2. The system as recited in claim 1, whereinthe RF synchronization logic is configured to enable communicationbetween the first RF generator and the second RF generator, and whereinthe first RF generator is configured to operate as a master RFgenerator, and wherein the second RF generator is configured to operateas a slave RF generator responsive to signals received from the first RFgenerator, and wherein the first RF generator is configured to directthe second RF generator when to output RF signals to the substrateholder.
 3. The system as recited in claim 1, wherein the RFsynchronization logic is implemented as a combination of hardware andsoftware.
 4. The system as recited in claim 1, wherein the RFsynchronization logic is implemented in each of the first RF generatorand the second RF generator.
 5. The system as recited in claim 1,wherein the RF synchronization logic is configured to direct the firstRF generator to operate in a continuous wave mode to supply RF signalsin a continuous manner to the substrate holder.
 6. The system as recitedin claim 5, wherein the RF synchronization logic is configured to directthe second RF generator to operate in a pulsed manner.
 7. The system asrecited in claim 6, wherein operation of the second RF generator in thepulsed manner includes operation of the second RF generator to supply RFsignals to the substrate holder during a high phase of a pulse.
 8. Thesystem as recited in claim 7, wherein the first RF generator and thesecond RF generator are configured to respectively supply RF signals tothe substrate holder in an additive manner during the high phase of thepulse.
 9. The system as recited in claim 1, further comprising: a coilassembly; and a primary RF generator connected to supply RF power to thecoil assembly, the coil assembly configured to generate anelectromagnetic field in response to the RF power to generate theplasma.
 10. The system as recited in claim 9, wherein the RFsynchronization logic is configured to direct the primary RF generatorto supply a low RF power to the coil assembly and simultaneously directboth the first RF generator and the second RF generator to supply RFsignals to the substrate holder to generate a high bias voltage at thesubstrate holder during a first process state of the two alternatingprocess states.
 11. The system as recited in claim 10, wherein the RFsynchronization logic is configured to direct the primary RF generatorto supply a high RF power to the coil assembly and simultaneously directthe second RF generator to not supply RF signals to the substrate holderand direct the first RF generator to supply RF signals to the substrateholder to generate a low bias voltage at the substrate holder during asecond process state of the two alternating process states, wherein thelow bias voltage is greater than zero.
 12. The system as recited inclaim 11, wherein the low bias voltage is low enough to avoidion-induced removal of a mask material from the substrate.
 13. Thesystem as recited in claim 11, wherein the RF synchronization logic isconfigured to control a duration of the first process state and aduration of the second process state, the RF synchronization logicconfigured to direct a change from the first process state to the secondprocess state following completion of the duration of the first processstate, the RF synchronization logic configured to direct a change fromthe second process state to the first process state following completionof the duration of the second process state, the RF synchronizationlogic configured to continue directing change from the first processstate to the second process state and from the second process state tothe first process state for an overall period of time.
 14. The system asrecited in claim 13, wherein the duration of the first process state isdifferent than the duration of the second process state.
 15. The systemas recited in claim 13, wherein the duration of the first process stateis substantially equal to the duration of the second process state. 16.The system as recited in claim 13, wherein the duration of the firstprocess state is less than ten percent of a sum of the duration of thefirst process state and the duration of the second process state. 17.The system as recited in claim 13, wherein the RF synchronization logicis configured to direct the primary RF generator to supply the low RFpower to the coil assembly within a range extending from about 50 Watts(W) to about 2000 W.
 18. The system as recited in claim 13, wherein theRF synchronization logic is configured to direct the primary RFgenerator to supply the high RF power to the coil assembly within arange extending from about 2000 W to about 5000 W.
 19. The system asrecited in claim 13, wherein the RF synchronization logic is configuredto direct the primary RF generator to supply RF signals of higherfrequency to the coil assembly during the first process state, andwherein the RF synchronization logic is configured to direct the primaryRF generator to supply RF signals of lower frequency to the coilassembly during the second process state.
 20. The system as recited inclaim 19, wherein the RF signals of higher frequency have a frequencywithin a range extending from about 13 megaHertz (MHz) to about 60 MHz,and wherein the RF signals of lower frequency have a frequency within arange extending from about 400 kiloHertz (kHz) to about 2 MHz.